Refactor data sources to throw exceptions instead of returning Optionals

[sebastian-peter]

Currently, if reading grid, participant components etc., an empty Optional is returned, accompanied by some error log messages that are writting into console, but which might be missed.

Refactor methods such as CsvSystemParticipantSource#getSystemParticipants, CsvRawGridSource#getGridData etc. to no longer return Optionals, but throw appropriate exceptions.